The purpose of kidney dialysis is to remove waste materials from blood that are normally removed by properly functioning kidneys. During dialysis, blood and dialysate are passed through a dialyzer on opposite sides of a membrane within the dialyzer. The dialysis system is maintained under specific pressure and flow conditions such that blood waste compounds pass through the membrane from the blood side to the dialysate side where they can be safely removed. Water molecules also can pass through the membrane during dialysis. The removal of some water from the blood can be beneficial to patients under certain circumstances. In other circumstances, however, it is desirable to replace water lost across the membrane during dialysis.